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CHAPTER 1

INTRODUCTION

1.1 POLYMER

The word polymer is derived from the classical Greek words poly

(meaning many) and meres meaning parts. Simply stated, a polymer is a

large molecule (macromolecule) composed of repeating structural units

typically connected by covalent chemical bonds. Certain polymers, such as

proteins, cellulose and silk, are found in nature. While many others, including

polystyrene, polyethylene and nylon, are produced only by synthetic routes.

The birth of polymer science may be traced back to the mid

nineteenth century. In the 1830s, Charles Goodyear developed the

vulcanization process that transformed the sticky latex of natural rubber to a

useful elastomer for tire use. In 1847, Christian F.Schonbein reacted cellulose

with nitric acid to produce cellulose nitrate. This was used in the 1860s as the

first man made thermoplastic, celluloid. In 1907, Leo Baekeland produced

Bakelite (phenol formaldehyde resin) and glyptal (unsaturated polyester resin)

was developed as a protective coating resin by General Electric in 1912.

By the 1930, researchers at Du Pont in the United States had

produced a variety of new polymers including synthetic rubber and more

exotic materials such as nylon and teflon. In 1938, Dow had produced several

tons of polystyrene and in 1939, polyethylene (low density) was made for the

first time by scientists at ICI in England. Efforts to develop new polymeric

materials, particularly synthetic rubber, were intensified during World War II,

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when many naturally occurring materials such as Hevea rubber were in short

supply. In the 1950s, Ziegler and Natta independently developed a family of

stereo specific transition-metal catalysts that lead to the commercialization of

polypropylene as a major commodity plastic. The 1960s and 1970s witnessed

the development of a number of high performance polymers that could

compete favourably with more traditional materials, such as metals, for

automotive and aerospace applications. Yet, today polymer dimensions are

neglected no more, for industries associated with polymeric materials employ

more than half of all American chemists and chemical engineers.

Products made from plastics and rubber materials are based on

polymers and contribute strongly to the national economy not least in terms of

performance, reliability, cost effectiveness and high added value. Among the

many reasons why polymers are widely used, two stand out. First, polymers

operating in a variety of environments have useful range of deformability and

durability which can be exploited by careful design. Secondly, polymers can

often readily, rapidly and at an acceptable (low) cost be transformed into

usable products having complicated dimensions. Moreover, the volume of

polymers used in the western economy already exceeds that of metals.

Most polymers can be classified as either thermoplastic or

thermoset, a label which describes the strength of the bonds between adjacent

polymer chains within the structure. In thermoplastics, the polymer chains are

only weakly bonded (Vander Waals forces). The chains are free to slide past

one another when sufficient thermal energy is supplied, making the plastic

formable and recyclable. In thermosets, adjacent polymer chains form strong

cross links. When heated, these cross links prevent the polymer chains from

slipping past one another. As such, thermosets do not reflow once they are

cured (i.e. once the cross links form). Instead, thermosets can suffer chemical

degradation (denaturing) if reheated excessively.

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However, the following Tables (1.1 and 1.2) provide thumbnail

sketches of the main thermoset polymer types, in terms of characteristics,

limitations and properties.

Table 1.1 Characteristics and limitations of some typical thermosetting

resins

S.No Resin type Characteristics Limitations

1. Phenolic Very good thermal stability,

Good fire resistance, Good

electrical properties.

Color limitation,

Alkali resistance.

2. Polyester Wide choice of resins, Easy

to cure at room temperature

and elevated temperature,

Good chemical resistance,

Good electrical properties.

Emission of styrene,

Shrinkage on curing,

Flammability.

3. Vinyl ester Good fatigue resistance,

Very good chemical

resistance, Good toughness.

Emission of styrene,

Shrinkage on curing,

Flammability.

4. Epoxy Very good chemical

resistance, Good thermal

properties, Low shrinkages

on curing.

Long cure cycle,

Best properties

obtained only with

cure at elevated

temperature.

5. Silicone Very good thermal and

electrical properties,

Excellent chemical

resistance, Resistant to

hydrolysis, Oxidation

resistance and non-toxic,

Good fire properties (self

extinguishing).

Adhesion, Long

cure cycle,

Commonly cured at

elevated

temperature.

6. Polyurethane Very good chemical

resistance, Very high

toughness (impact), Good

abrasion resistance.

Isocyanates as

curing agents, Color,

Anhydrous curing

conditions.

7. Polyimide and

polyamide-

imide

Excellent thermal stability,

Good electrical and fire

properties.

Color, Arc

resistance, Acid and

alkali resistance.

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Table 1.2 Properties of cast thermoset resins

S.

No.Property (unit) Polyester

Vinyl

esterEpoxy Silicone Polyimide

1. Density (g/cm) 1.19-1.20 1.12-1.30 1.21-1.24 1.42-1.51 1.72-1.81

2. Tensile strength

(MPa)

55-62 80-96 76-104 38-56 94-100

3. Tensile elongation

(%)

1.80-2.10 5.0-6.30 11.46-

13.20

100-400 90-94

4. Tensile modulus

(GPa)

1.30-1.60 3.30-3.60 1.73-2.10 3.50-4.70 2.50-2.80

5. Flexural strength

(MPa)

100-120 145-151 128-160 97-120 156-173

6. Flexural modulus

(MPa)

2400-2600 3100-3800 3100-4200 4200-4520 4300-4500

7. Heat distortion

temperature ( C)

67-72 102-110 81-92 118-120 122-126

Polymeric materials are used in and on soil to improve aeration,

provide mulch, promote plant growth and health. Many biomaterials,

especially heart valve replacements and blood vessels, are made of polymers

like dacron, teflon and polyurethane.

Plastic containers of all shapes and sizes are light weight and

economically less expensive than the more traditional containers. Clothing,

floor coverings, garbage disposal bags and packaging are other polymer

applications.

Automobile parts, windshields for fighter planes, pipes, tanks,

packing materials, insulation, wood substitutes, adhesives, matrix for

composites and elastomers are some applications in the industrial market.

Playground equipment, various balls, swimming pools and

protective helmets are often produced from polymers.

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1.2 COMPOSITES

The wide spectrum of properties available to polymeric materials

has afforded numerous practical applications ranging from common

household goods to biomedical materials and aerospace components. Often,

however, the inherent properties of the polymer alone are insufficient to meet

the structural demands of an application. As a result, blending with a stronger

or stiffer material is necessary to improve upon the mechanical performance

of the pristine polymer.

A composite structure is a combination of two or more different

components resulting in a material having better performance than each

individual constituent. Composite structures have been widely applied to the

automotive, aerospace and defense industries. Polymer matrix composites are

(PMCs) lightweight compared to the traditionally used metallic materials,

thereby enabling airplanes, missiles and spacecraft to operate with less fuel or

increased payload. The majority of PMCs used in aerospace and aeronautics

applications utilize a high performance resin matrix with carbon fiber

reinforcement.

By principle, polymer composite is a combined material created by

the assembly of two or more components viz., selected filler or reinforcing

agent (silicates, aluminates and fibers) and a compatible binder (resin) in

order to obtain specific characteristics and properties. In the broadest form,

composites are the result of embedding high strength, high stiffness fibers of

one material in a surrounding matrix of another material. The fibers are

principal load carrying members whereas the surrounding matrix keeps them

in the desired location and orientation and act as a load transfer medium

between them. Various properties that can be improved by the use of

composites are mechanical strength, stiffness, corrosion, fatigue life and high

temperature resistance properties.

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1.2.1 Types of Polymer Composites

A composite material has at least one continuous phase (binding

matrix) and one or more dispersed phases (fillers and/or reinforcements).

Composites can be classified based on the type of binding matrix

as, Polymer Matrix Composites (PMC), Ceramic Matrix Composites (CMC)

and Metal Matrix Composites (MMC).

PMCs are constructed of components such as glass, carbon, aramid

or boron fiber and/or mineral fillers bound together by an organic polymer

matrix.

CMCs are made by inserting a second phase in the form of fibers,

chopped fibers, small discontinuous whisker platelets or particulates in a

continuous phase of ceramic material.

MMCs are made by inserting a second phase in the form of fibers,

whiskers (or) particulates in a continuous phase of metal.

Depending on the type of dispersed phase, composites are classified

as fiberous, laminated and particulate.

Fiberous composites consist of fibers (short or discontinuous and

randomly arranged) in a matrix. The fibers may be continuous, e.g. long fibers

or ribbons; these are embedded in the polymer in regular geometric

arrangements that extend throughout the dimensions of the product. Familiar

examples are the well-known fiber-based thermoset laminates that are usually

classified as high performance polymer composites.

On the other hand, the fibers may be discontinuous (short), for

example, short fibers (say < 3 cm in length), flakes, platelets, spheres or

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irregulars; these are dispersed throughout the continuous matrix. Such

systems are usually based on a thermoplastic matrix and are classified as

lower performance polymer composites compared to their counterparts with

continuous additives.

Laminated composites are formed by bonding together layers of

planar reinforcement with resin.

In particulate composites, the dispersed phase is often spherical or

at least has dimensions of similar order in all directions. Calcium carbonate,

talc and mica filled polymers, solid rocket propellants etc., are some examples

for particulate composites.

1.2.2 Composites Basic Ingredients

1.2.2.1 Reinforcements

Reinforcements comprise of fiberous materials that are used to

strengthen the cured resin systems. Virtually any fiberous material may be

used, although in practice the list is quite small. Fiber length may vary from

about 3 mm up to several hundred meters. Glass fibers are the predominately

used reinforcements among all. Apart from this, polyester, acrylic, carbon and

aramid based fiber (and natural fiber such as sisal, coir, hemp and banana

fiber) are also used for making composites.

1.2.2.2 Matrix

The reinforcing constituent such as fiber can resist only tension

unless they are bound together through a binding media which is called as

matrix. Among other things, the main purpose of a matrix is to support,

protect and transfer load from fiber to fiber. Always, the matrix is of lower

stiffness, strength and density as compared to the reinforcement. But the

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combination of the constituents in a correct proportion can have considerable

high strength yet maintaining low density.

Generally, the matrix is essentially the binding and shaping

component in composite materials. Its properties determine to a large extent,

the process conditions for the manufacture of composite materials and the

important operating characteristics like resistance to environmental effects,

working temperature, fatigue strength and specific strength etc. Phenolic,

unsaturated polyester, vinyl ester resin, epoxy resin, polyimide resins and

other thermoplastic resins like poly (ether ether ketone) (PEEK),

polyphenylene sulphide (PPS) are some of the common resins used for

making polymer matrix composites.

1.2.2.3 Fillers

Fillers are solid additives, which are added to polymers to increase

bulk or improve properties, often while lowering costs but increasing the

specific gravity. They are generally inorganic and less frequently organic.

Functional fillers produce specific improvements in certain mechanical and

physical properties. The extent of property enhancement depends on many

factors including the aspect ratio of the filler, its degree of dispersion and

orientation in the matrix and the adhesion at filler-matrix interface (Makadia

2000, Premphet and Horanont 1999).

Based on the mutual interaction between filler and matrix, it is

possible to classify fillers into two groups.

Inactive

Active

Classification is arbitrary since it is based not only on the difference

in the chemical composition but also on the characteristic of filler particle

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surface, particle shape and size, the treatment of the surface by coupling

agents. The fillers activity is governed by the mutual adhesion of the polymer

and filler; it corresponds to the physico-chemical character of the polymer-

filler interfaces which determine the extent of sorption processes on the solid

surfaces as well as the type of polymer filler bonds.

The action of fillers can be contributed to several mechanisms:

some fillers form chemical bonds with the material being reinforced; for

example, carbon black produces cross linking in elastomers by means of

radical reactions (Ketan 2002). The polymer segments attached to filler

surfaces by primary or secondary valence bonds in turn cause a certain

immobilization of adjacent segments and circumstances permitting an

orientation of the polymer matrix. Stiffening, lower deformability and higher

strength are due to this composite nature. Another mode of action of active

fillers results from the fact that when the polymer molecules are subject to

mechanical stress due to the absorption of energy, they can slide off the filler

surface. Therefore, the impact energy can uniformly distribute increasing the

impact resistance.

Inorganic particles are widely used in polymers because they are

usually lower in cost. The frequently used fillers, e.g. glass beads, glass fibers,

calcium carbonate, silica, talc and mica usually form microcomposites with

limited improvement in properties (Makadia 2000, Ray and Okamoto 2003).

Inactive fillers function as stress concentration agents. They initiate

the fracture in the polymer mass and lower the energy necessary for the

physical destruction of compounded plastics. Powdered fillers are known as

low aspect ratio fillers and are mainly of inorganic origin. But it is observed

that the filler which function as inactive fillers can act as reinforcing filler in

the other matrix, depending upon the nature of both filler and the polymer.

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A number of types of fillers are generally recognized in polymer

technology and these are summarized in Figure 1.1.

Figure 1.1 Types of fillers

Fillers may be classified on the basis of their chemical composition,

nature, function and particle morphology as shown in Table 1.3.

Table 1.3 Classification of fillers

By particle

morphologyBy composition By function

Crystalline

Fibers

Platelets

Polyhedrons

Irregular

Masses

Whisker

Amorphous

Fibers

Flakes

Solid spheres

Hollow sphere

Irregular masses

Inorganic

Carbonates

Fluorides

Hydroxides

Metals

Oxides

Silicates

Sulfates

Sulfides

Organic

Cellulose

Fatty acids

Lignin

Polyalkene

Polyamides

Polyaromatics

Polyesters

Proteins

Reinforcement

Cost reduction

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When employed in elastomeric system, it is commonly observed

that the finer the particle size the higher the values of such properties as

tensile strength, modulus and hardness. Coarser particles will tend to decrease

the strength of the pure cast resin but if the particle size is sufficiently fine

there is an enhancement in the above mentioned properties (at least up to an

optimum loading of filler) and the phenomenon is known as reinforcement

(Brydson 1975).

1.2.3 Characteristics of Fillers

The action of various fillers on a given polymer differs

significantly. However, the fillers vary markedly in their interaction with

polymers, there are a number of filler properties which strongly influence the

reinforcing characteristics of each type. Important parameters are follows.

1. Particle size, size distribution and content. These parameters

will determine. e.g. the inter particle distance

2. Particle shape and surface structure

3. Mechanical properties of the mineral

4. Compounding and molding methods used

5. Bond strength between mineral and polymer. This will be

influenced by the type of dispersion aid or coupling agent

used

6. Polymer properties, e.g. ductile polymers will behave

differently from the brittle ones.

1.2.4 Polymer Filler Interaction

The fundamental aspects of polymer-filler interaction have been

studied in a number of polymers (Leigh and Dougmore 1961, Blanchard

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1961, Alter 1965, Guth 1945). Several possible types of polymer-filler

interaction can be visualized.

i. Simple physical inclusion of the filler particle (and

agglomerates) in a matrix of non polar polymer. In this case,

the filler would be expected to weaken the polymer, since it

would function only as a diluent.

ii. Physical inclusion of the filler within a polymer matrix along

with wetting of the filler surface by the polymer. Here, some

stiffening along with the usual rise in tensile strength and drop

in elongation might be expected.

iii. Definite physical adhesion of the polymer to the surface of the

filler particles (or fibers). In this case, a marked overall

strengthening effect would be expected.

iv. The establishment of true chemical bonds between the

polymer and individual particles of filler; for example the

reinforcement of rubber by carbon black.

Since, it is generally found that the presence of any appreciable

quantity of filler causes stiffening of the polymer system, it is likely that the

first case is quite rare. The second case probably includes most fillers used for

cost reduction in thermoplastics and as pigment or extender in coatings. The

third and fourth cases are important where strengthening action through the

use of the filler is the primary goal.

Both glass fiber reinforced thermosetting plastic and carbon black

reinforced rubber are specific examples. Whether strong physical adhesion or

actual chemical bonding is involved has not been established for either of

these systems and the mechanism is obscure.

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The degree to which some positive form of polymer filler

interaction is present in a given system is influenced by a number of factors

viz., geometry of the particle (surface area, particle size, shape and porosity),

its volume fraction, the ease with which filler surface is wetted and the ease

with which the filler is dispersed in the polymer.

1.2.5 Effect of Addition of Mineral Fillers in Thermosetting

Polymers

Thermosetting or heat-hardening resins are those which cure on

heating to form cross links between individual chains and give infusible

brittle products. In this form the mechanical properties of those resins,

particularly impact strength, are generally poor and it may be altered by the

addition of fillers.

General purpose unsaturated polyester resin (GPR) exhibits

brittleness in the unfilled state. Reinforcing and non-reinforcing fillers are

generally employed either to improve the mechanical properties or to assist

processing and to reduce cost. Non-reinforcing fillers mostly used with GPR

are clays (Richardson 1977), silica (Burns 1977), calcium carbonate (Bassford

1978), barium sulphate, mica (Bajaj et al 1992) and talc (Tiegi et al 1998).

They are used mainly for cheapness and to give specific effects in the cured

system. Normally fillers of this type affect the setting. All the fillers tend to

increase the viscosity of the resin and this may reduce the volumetric

shrinkage (Lindmeyer 1951) during curing. Fillers may also be used to give

improved surface finish and finely powdered fillers are often used alone in gel

coats. Powdered mineral fillers tend to increase the compressive strength and

hardness of the product. Fillers can also play an important part in the handling

characteristics of a resin mix.

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The quantities of filler used in epoxy type resin system vary

according to the type of filler used, the handling characteristics desired and

the properties required. Fluffy fillers like asbestos or uncompressed silica

produce large increase in viscosities at filler loading of 25 phr. Fillers such as

talc (Tiegi et al 1998), nickel powder and aluminium may be used in

quantities up to 200 phr and heavier fillers in quantities of the order of 300

phr.

Fillers tend to slow the curing rate and reduce the exotherm because

of their diluting effect (Lee and Reville 1982). Fillers such as mica powder

(Bajaj et al 1992) and glass improve electrical properties, particularly arc

resistance in epoxy resin. Solvent resistance of epoxy resin filled with 30%

clay, 50% calcium carbonate, 50% microsil and 65% zirconium silicate are

better than that of the pure resin.

1.2.6 Applications and Trends of Composites

Global demand for fillers/reinforcing fillers, including calcium

carbonate, aluminum trihydrate, talc, kaolin, mica, wollastonite, glass fiber,

aramid fiber, carbon fiber and carbon black for the plastics industry is

estimated to be about 15 million tons. Primary end-use markets are building/

construction and transportation, followed by appliances and consumer

products, furniture, industrial/machinery, electrical/electronics and packaging

comprise smaller market segments. Flexural modulus and heat resistance are

the two critical properties of plastics that are enhanced by the inclusion of

performance minerals. Automotive exterior parts, construction materials,

outdoor furniture and appliance components are examples of applications

benefiting from enhanced flexural modulus. Automotive interior and

underhood parts, electrical connectors and microwaveable containers are

examples of applications requiring high temperature resistance.

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Statistics suggested that demand for inorganic minerals for use in

plastics, of about 4 million tons per annum, with an average annual growth

forecasted to be about 4.2%. Data (not including glass products, natural fibers

or nanofillers but including titanium dioxide) indicate the highest demand for

ground calcium carbonate (60% of the total), followed by titanium dioxide

(13%), aluminum trihydrate (10%) and talc (10%). Kaolin, mica, wollastonite

and barites have a much smaller share of the market. When glass and natural

fibers are included in the statistics, calcium carbonate accounts for 40% of the

total market, glass for 30% and other mineral fillers and natural fibers for

20%. Combinations of fillers are also often used to impart specific combined

properties not attainable with single filler. Among polymers, polyvinyl

chloride is still the plastic with the highest filler usage, followed by

polyolefins, nylons and polyesters (Xanthos 2005).

1.3 NANOCOMPOSITES

A polymer nanocomposite is a polymer matrix with a reinforcing

phase consisting of particles with atleast one dimension in the nano-sized

regime. In the past decade, extensive research has focused on polymer

nanocomposites in hopes of exploiting the unique properties of materials in

the nano-sized regime.

The term polymer nanocomposite broadly describes any number

of multicomponent systems, where the primary component is the polymer and

the filler material has at least one dimension below 100 nm. Polymer

nanocomposites are generally light weight, require low filler loading, are

often easy to process and provide property enhancements extending orders of

magnitude beyond those realized with traditional composites.

In mechanical terms, nanocomposites differ from conventional

composite materials due to the exceptionally high surface to volume ratio of

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the reinforcing phase and/or its exceptionally high aspect ratio. The

mechanical properties of the composites filled with micron sized filler

particles are inferior to those filled with nanoparticles of the same filler. In

addition, the physical properties, such as surface smoothness and barrier

properties cannot be achieved by using conventional micron sized particles.

The reinforcing material can be made up of particles (e.g. minerals), sheets

(e.g. exfoliated clay stacks) or fibers (e.g. carbon nanotubes or electrospun

fibers). The area of the interface between the matrix and reinforcement

phase(s) is typically an order of magnitude greater than for conventional

composite materials. The matrix material properties are significantly affected

in the vicinity of the reinforcement. Polymer nanocomposites properties are

related to degree of thermoset cure, polymer chain mobility, polymer chain

conformation, degree of polymer chain ordering or crystallinity can all vary

significantly and continuously from the interface with the reinforcement into

the bulk of the matrix.

1.3.1 Background

Polymer systems are widely used because of their light weight,

design flexibility and processability. These systems, however, generally

exhibit less attractive mechanical properties such as low strength and low

elastic modulus as compared to metals and ceramics. One way to improve the

mechanical properties of these systems while maintaining their desirable

properties is by adding high-modulus reinforcing filler to make polymer

composites. Adding micro-sized inorganic filler particles to reinforce the

polymeric materials has been standard practice in the composite industry for

decades. However, in the case of micron sized fillers, above 20% volume

fraction is required to get optimal impact mechanical properties. At these high

concentrations, the filler can detrimentally impact other benefits of polymers

such as processability and appearance. Because of their small size, nano

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particles have an extremely high surface to volume ratio providing

significantly more surface area for bonding with the matrix than micro-sized

fillers (Zhoa et al 2008, Krishmoorthi et al 1996). Polymer nanocomposites,

consisting of a polymer matrix with nanofiller, have been predicted to be one

of the most beneficial applications of nanotechnology. Much research has

been focused on the preparation and thermal and mechanical characterization

of nanocomposites.

Although some research has shown great improvement of

mechanical properties for nanocomposites over those of micro-filled

composites, results have not been consistent. Additionally, the varying

polymer matrix/filler systems and varying preparation techniques do not

support establishing clear trends in polymer nanoparticle performance.

Current polymer models have not been able to consistently predict

the properties of nanocomposites. Polymer composite theories in the past

have relied on the idea that the modulus of a composite is a function of the

mismatch of properties of constituents, volume fraction, shape and

arrangement of inclusions and matrix-inclusion interface. Recent theories

have included the size of the filler particulate to predict the properties of

composites (Ciprari 2004).

1.3.2 Progress in Nanocomposites

Nanocomposites were first referenced as early as 1950 and

synthetic polymer-clay nanocomposites were first reported as early as 1961,

when Blumstein demonstrated polymerization of vinyl monomer intercalated

into montmorillonite clay and polyamide nanocomposites were reported as

early as 1976. However, it was not until researchers from Toyota Central

Research and Development Laboratories (CRDL) in Japan in the late 1980s

began a detailed examination of polymer/layered silicate clay mineral

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composites that nanocomposites became more widely studied in both

academic and industrial laboratories.

The total global market value for nanocomposites nears three

million pounds; of which two million pounds are nanoclay-reinforced

polyamides (nylon) produced by Unitika and Ube Industries in Japan for

automotive and packaging application respectively. The remaining one

million pounds are carbon nanotube-filled polyphenylene oxide/nylon alloy

produced in North America for automotive body parts. Each of these

developing product technologies is poised for strong growth over the next ten

years. Market projections show that the demand in each region will grow at

comparable rates. The market reached nearly 1.2 billion pounds in 2009, of

which one billion pounds will be nanoclay reinforced compounds and 160

million pounds will be carbon nanotube-filled products.

Nanocomposites technology is applicable to a wide range of

polymers, cutting across the materials classes of thermoplastics, thermosets

and elastomers. Over the next ten years, nanoclay composites of nearly 20

polymers are expected to be commercialized. Therefore, nanocomposites

technology is recognized as one of the promising avenues of technology

development for the 21st century. Nanocomposites are currently used in two

commercial applications: automotive under hood components and food

packaging. The goals are physical, mechanical and thermal properties

enhancement and reduced permeability. Nylon-based nanocomposites were

the first commercial materials to emerge and there is now a frenzy of activity

aimed at nano-reinforcing commodity thermoplastics such as polypropylene

(PP) and polyethylene terepthalate (PET). These end markets will continue to

be the primary outlets for nanocomposites over the next ten years (Ketan

2002).

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1.3.3 Types of Nanocomposites

In nanocomposites, at least one dimension of the dispersed particles

is in the nanometer range. One can distinguish three types of nanocomposites,

depending on how many dimensions of the dispersed particles are in the

nanometer range.

When the three dimensions are in the order of nanometers, we are

dealing with isodimensional nanoparticles (or simply nanoparticles). Carbon

black, silica, aluminium oxide, titanium dioxide, zinc oxide, silicon carbide,

polyhedral oligomeric sislesquioxanes (POSS) are examples for nanoparticle

fillers.

When two dimensions are in the nanometer scale and the third is

larger, an elongated structure is formed. Examples are carbon nanotubes,

carbon nanofibers, cellulose whiskers, boron nitride tubes, boron carbon

nitride tubes and gold or silver nanotubes.

The third type of nanocomposites is characterized by only one

dimension in the nanometer range. In this case the filler is present in the form

of sheets of one to a few nanometer thick to hundreds to thousands

nanometers long. This family of composites can be gathered under the name

of nanoplatelet based nanocomposites.

There are mainly three types of morphology dominating in the

nanocomposites: exfoliated, intercalated and phase separated. The

morphology governs the properties of the nanocomposites. Probability of

possible aggregation, intercalation or exfoliation of nanoparticles in a polymer

matrix depends on many factors such as nature of particle, nature of matrix,

different processing parameters and so on. Intercalation and exfoliation

mechanism of particle in the polymer matrix are shown in Figure 1.2.

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Figure 1.2 Scheme of different types of composites arising from the

interaction of layered silicates and polymers

In phase separated composite, the polymer is unable to intercalate

between the silicate sheets. In an intercalated structure, the organic

component is inserted between the layers of the nanoparticles such that the

inter layer spacing is expanded but spatial relationship to each of the layers

remains unaffected. In an exfoliated morphology, the nanoparticle layers

become completely separated rendering a well distributed individual layers

throughout the polymer matrix (Ray et al 2003).

1.3.4 Nanoparticles

Novel properties of nanocomposites can be obtained by

successfully imparting the characteristics of parent constituents to a single

material. These materials differ from both the pure polymers and the

inorganic nanoparticles in some physical and chemical properties.

Encapsulation of inorganic nanoparticles inside the shell of polymers is the

most popular and interesting approach to synthesize nanocomposites.

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Nanoparticles can be defined as materials and systems whose

structures and components exhibit novel and significantly improved physical,

chemical and biological properties, phenomena and processes due to their

nanoscale size i.e. in a range of 1-100 nm. At present, there is a wide

spectrum of technological approaches capable of producing nanoparticles and

simple nanostructures, however, none of them can be considered as an ideal

and generally acceptable tool. The nanoparticles listed above differ in

chemistry, morphology, aspect ratio and aggregate size. The nanoparticles

chosen for dispersion in a resin are dependent on the intended application.

However, realization of significant enhancement in properties with any of the

nanomaterials requires that the nanoparticle is well dispersed throughout the

matrix. Furthermore, it is desirable that the particle bond with the matrix. The

nanoparticles are made from different sources and include the following types

Zinc oxide, alumina, calcium carbonate, titanium dioxide, etc.

Silica

Silicon carbide, carbon black, polyhedral oligomeric

silsesquioxanes (POSS)

1.3.5 Role of Nanoparticles

A general conclusion has been drawn that nanocomposites show

much improved mechanical properties over their micro-sized similar systems.

Because of their small size, nanoparticles have a high surface to volume ratio

and provide high energy surfaces. An expected result of embedding

nanoparticles into a polymer matrix is enhanced bonding between the polymer

matrix and filler, resulting from the nanoparticles high interfacial energy.

Polymer composite theory predicts that improved bonding between polymer

and matrix leads to improved mechanical properties. Despite these

predictions, however, mechanical testing of nanocomposites has shown mixed

22

results. Some experimental data has shown that reduced particle size

improves mechanical properties, specifically elastic modulus. Other studies

have shown that elastic modulus decreases with reduced dimensionality. No

clear conclusions have been made regarding trends in the mechanical

properties of polymer nanocomposites (Kar et al 2008). Although studies

have focused on many different matrix-filler systems, a common feature of all

polymer composites is the existence of a phase border between the matrix and

filler and the formation of an inter phase layer between them. The properties

of the inter phase can differ dramatically from the bulk and influence the

mechanical properties of the composite. To explain the impact on properties,

a number of theories have been applied to the study of inter phase.

1.3.6 Review on Inter Phase

An area of polymer composite structure that has always garnered

attention is the region directly near the interface of the polymer matrix and the

filler. Despite the huge variety of polymer composite systems, a common

thread among all the systems is the existence of a phase border between the

matrix and filler and the formation of an inter phase layer between them. As

seen in Figure 1.3 the inter phase layer extends well beyond the adsorption

layer of the matrix chains bound to the filler surface. The structure of the inter

phase is different from that of both the filler and the matrix phases and it

varies depending on the distance from the bound surface. Because of the

differences in structure, inter phase properties can differ dramatically from the

bulk polymer. The inter phase is important to the mechanical properties of the

composite because its distinct properties control the load transfer between

matrix and filler. The concept of inter phase is not unique to nanocomposites

but due to the large surface area of nanoparticles, the inter phase can easily

dominate the properties of nanocomposites.

23

Vollenberg and Heiken proposed the role of inter phase, they

explained an increase in modulus, yield and tensile strength in composite

systems by defining the properties of the inter phase region.

Figure 1.3 Schematic of inter phase area between filler and the

polymer matrix

According to the theory, if a polymer adhered to a filler particle

surface where modulus is high, there would be an area of high density and

thus, high modulus next to the particle. The polymer portion in the area just

outside the particle will move toward the high density area, leaving an area of

low density and low modulus just away from the high density area.

Vollenberg and Heiken proposed that for large particles, the size of the low

density area will be relatively large which will lessen the impact of the higher

modulus filler. But in a nanocomposite there are many more particles required

to achieve the same volume fraction, so the particles are much closer together.

If the particles are close enough the higher density bound layer of the polymer

will comprise a larger fraction of the matrix, so the lower modulus will have

little effect on the system. This logic suggests that nanocomposites will have

improved mechanical properties over large scale composites.

Generally, composite components should interact either chemically

or physically, with the matrix to create a strong interface. In all cases, the

interface between the composite components plays a defining role in the

overall material properties. This becomes critical as the available interfacial

24

area is increased by the dispersion of nanosized particles throughout the

polymer matrix. Creating a strong bond with the matrix requires an

understanding of the interactions taking place at the matrix/nanoparticle

interface (Ciprari 2004).

1.3.7 Nanoparticle Dispersion

A critical factor in nanocomposite properties is the dispersion of the

nanoparticles in the polymer matrix. Dispersion of inorganic nanoparticle

filler in a thermoplastic is not easily achieved because nanoparticles have a

strong tendency to agglomerate to reduce their surface energy. Some studies

have used adsorbed polymers to sterically stabilize nanoparticle dispersions

limiting formation of flocculants and aggregates. Other studies have used

different approaches to in-situ polymerization to achieve good particle

dispersion. Although the studies presented above utilized a wide range of

sample preparation techniques, no technique proved significantly better than

any others.

1.3.8 Manufacture of Clay Nanocomposites

Polymer/layered silicate nanocomposites are currently prepared in

four ways:

In-situ intercalative polymerization

Melt intercalation

Solution blending

Template synthesis

In situ intercalative polymerization: In this technique, the layered

silicate is swollen within the liquid monomer (or a monomer solution) so as

the polymer formation can occur in between the intercalated sheets.

25

Polymerization can be initiated either by heat or radiation, by the diffusion of

a suitable initiator or by an organic initiator or catalyst fixed through cationic

exchange inside the interlayer before the swelling step by the monomer.

Melt intercalation: The layered silicate is mixed with the polymer

matrix in the molten state. Under these conditions and if the layer surfaces are

sufficiently compatible with the chosen polymer, the polymer can crawl into

the interlayer space and form either an intercalated or an exfoliated

nanocomposite. In this technique, no solvent is required.

Solution blending: Solution blending is based on a solvent system

in which the polymer or pre polymer is soluble and silicate layers or other in

the inorganic nanofiller is swellable. First, polymer solution is prepared and

the inorganic nanoparticles are swollen in a solvent. Then the polymer and the

nanoparticle suspension are mixed, thus, nanosized particles can be dispersed

in the polymer matrix resulting in polymer nanocomposites. In the case of

layered silicate polymer nanocomposites, the polymer chains intercalate and

displace the solvent within the interlayer of the silicate. Upon solvent

removal, the dispersed structure remains.

Template synthesis: This technique, where the silicates are formed

in situ in an aqueous solution containing the polymer and the silicate building

blocks, has been widely used for the synthesis of double-layer hydroxide-

based nanocomposites. But is far less developed for layered silicates. In this

technique, based on self-assembly forces, the polymer aids the nucleation and

growth of the inorganic host crystals and gets trapped within the layers as

they grow.

Direct polymer melt intercalation is the most attractive method and

a large number of research and development works focused on this because of

its low cost, high productivity and compatibility with current processing

26

techniques (i.e. extrusion and injection molding) (Alexandre and Dubois

2000). Besides that, direct polymer melt intercalation is an effective

technology for polyolefin-based nanocomposites, especially for

polypropylene based nanocomposites.

1.3.9 Areas of Applications

The number of commercial applications of nanocomposites has

been growing at a rapid rate. It has been reported that at 2010, the worldwide

production is exceeded 6,00,000 tonnes and is set to cover the following key

areas in the next five to ten years:

Drug delivery systems

Anti-corrosion barrier coatings

UV protection gels

Lubricants and scratch free paints

New fire retardant materials

New scratch/abrasion resistant materials

Superior strength fibers and films

Improvements in mechanical property have resulted in major

interest in nanocomposite materials in numerous automotive and

general/industrial applications. These include potential for utilization as

mirror housings in various vehicle types, door handles, engine covers, intake

manifolds and timing belt covers. More general applications currently being

considered include usage as impellers and blades for vacuum cleaners, power

tool housings, mower hoods and covers for portable electronic equipment

such as mobile phones, pagers etc.

27

1.3.9.1 Food packaging

Development of a combined active/passive oxygen barrier system

for polyamide-6 materials is underway at various laboratories across the

world. Passive barrier characteristics are provided by nanoclay particles

incorporated via melt processing techniques whilst the active contribution

comes from an oxygen-scavenging ingredient.

Increased tortuosity provided by the nanoclay particles essentially

slows transmission of oxygen through the composite and drives molecules to

the active scavenging species resulting in near zero oxygen transmission for a

considerable period of time. Such excellent barrier characteristics have

resulted in considerable interest in nanoclay composites in food packaging

applications, both flexible and rigid.

Specific examples include packaging for processed meats, cheese,

confectionery, cereals and boil-in-the-bag food, also extrusion-coating

applications in association with paperboard for fruit juice and dairy products,

together with co-extrusion processes for the manufacture of beer and

carbonated drinks bottles. The use of nanocomposite packaging would be

expected to enhance the shelf life of many types of food considerably.

1.3.9.2 Fuel tanks

The ability of nanoclay incorporation to reduce solvent

transmission through polymers such as polyamides has been demonstrated.

Available data reveals significant reductions in fuel transmission through

polyamide6/66 polymers by incorporation of nanoclay filler. As a result,

considerable interest is now being seen in these materials as both fuel tank

and fuel line components for cars.

28

1.3.9.3 Films

The presence of filler incorporation at nano-levels has also been

shown to have significant effects on the transparency and haze characteristics

of films. The nanoclay incorporation has been shown to significantly enhance

transparency and reduce haze.

Similarly, nano-modified polymers have been shown, when

employed to coat polymeric transparency materials, to enhance both

toughness and hardness of these materials without interfering with light

transmission characteristics.

1.3.9.4 Environmental protection

Available data indicate that significant reduction of water

absorption in a polymer could be achieved by nanoclay incorporation. Similar

effects could also be achieved with polyamide-based nanocomposites.

Specifically, increasing aspect ratio diminishes substantially the amount of

water absorbed, thus indicating the beneficial effects likely from nanoparticle

incorporation compared to microparticle loading.

Hydrophobicity enhancement would clearly promote both

improved nanocomposite properties and diminish the extent to which water

would be transmitted through to an underlying substrate. Thus applications in

which contact with water or moist environments is likely could clearly benefit

from materials incorporating nanoclay particles.

1.3.9.5 Flammability reduction

National Institute of Standards and Technology in the US has

demonstrated the extent to which flammability behaviour could be restricted

in polymers such as polypropylene with as little as 2% nanoclay loading. In

29

particular heat release rates, as obtained from cone calorimetry experiments,

were found to diminish substantially by nanoclay incorporation.

1.4 UNSATURATED POLYESTER RESIN

1.4.1 Synthesis

Unsaturated polyester resin (UPR) is prepared by the reaction

between dihydric alcohols and unsaturated dibasic acids (maleic anhydride or

fumaric acid) (or) anhydride and saturated diacid/anhydride (phthalic

anhydride). The resultant polyester contains reactive double bonds along the

entire polyester chain, which become the site for cross linking. Even with

effective catalysts, they still require high temperatures and lengthy cure time

to complete cross linking. This can be avoided by the addition of reactive

monomers, such as styrene.

Figure 1.4 Saturated anhydrides/acid, unsaturated anhydrides/acid

and alcohols form polyester

1.4.2 Advantages

Unsaturated polyesters are one of the most important families of

thermosetting resins which can be formulated with a variety of properties

ranging from hard and brittle to soft and flexible. Their advantages are low

viscosity, fast cure time and low cost (Patel et al 1990). Even though slightly

inferior to epoxy resins, in overall properties, they are always the first choice

of a fiber reinforced plastics molder. It is easy to fabricate units with

30

polyesters in cold as well as hot conditions. It have a leading role in the

development of fiberglass reinforced products, having tremendous versability

and low cost. The use of UPRs in hand layup process, bulk and sheet molding

compounds results in composite materials that have high strengh, dimension

stability and very good surface quality. In addition, a wide variety of

modifications could be affected in polyesters by changing suitably the

monomers that go into its production so that exact polyester of specified

properties could be produced. They have many applications in automotive,

aircraft, electrical and appliance components as substitutes for traditional

materials.

1.4.3 Properties

In case of chemical resistant polyester one has to change phthalic

anhydride to isophthalic acid. Such a change incurs greater flexibility as well

as greater acid and solvent resistance in the resin. However, these resins are

easily attacked by alkalies which can be overcome by partially replacing the

glycol with bicyclic aliphatic diol like hydrogenated bisphenol A or

cyclohexane diol.

Some characteristics that one should look for in polyesters are

viscosity, non-volatile content, gel time and exotherm/thermal behaviour. The

finished properties tend to approach an optimum value when the molecular

weight of the base resin is 1000-1200 and the viscosity (200-300 cP). Most of

the standard commercial resins available have molecular weights above this

range. Any further variation in the properties cannot be based on variation in

molecular weight. Raising the styrene content above 40% for a general

purpose resin results in a decrease in flexural strength and modulus. With the

increase in styrene content even the tensile strength is affected.

31

Fillers are added to UPR system to reduce shrinkage after cure, for

improving molding characteristics and cured properties or to reduce the

overall cost of the system (Sarojadevi et al 1998). Almost any powdered

material can be used as a filler (Paauw and Pizzi 1994, Yu et al 1976, Lisaka

and Shibayma 1978, Wu 1988, Doului and Hoen 1994, Liu and Gilbert 1996,

Nikhil et al 2001, Sagi-Manna et al 1988, Vipulanandan and Dharmarajan

1989, Luo and Wong 2001, Guhanathan et al 2001, Navin and Nidhikhare

1999) the common ones being obtained from natural deposits (Marcovich et

al 1996). Of the several hundreds of fillers calcium carbonate, quartz and

silica, flours, talc and various clays are widely used (Katz and Mileswski

1978, Chand and Gautham 1993). Generally fillers affect the properties of

polyester compounds in several ways. The most important of these are the

modification of viscosity and rheology. Viscosity rises as the filler content is

increased.

1.4.4 Types of Unsaturated Polyester Resin

There are basically four different types of unsaturated polyester

resin solutions; they are the low profile dicyclopentadiene, orthopthalic,

isopthalic and vinyl ester. The type of resin produced is dictated by the raw

materials, which are used to make the polyester molecule and will result in

these products having different end properties and different associated costs.

While resins may fit into these different classes, many manufacturers blend

them together for their different products.

1.4.5 Curing of Unsaturated Polyester

The curing or cross linking of unsaturated polyester resin can be

achieved at room temperature by adding catalyst (or) initiators plus an

accelerator (or) promoter and at elevated temperature just by adding a catalyst

and heating. In order to achieve the optimum cure properties from the resin

32

system, the catalyst and accelerator must be chosen with care and correct

amount used. Supplier generally recommends particular catalyst and

accelerator combination, for use with each of their resins together with

proportions.

1.4.5.1 Catalyst

Catalyst used is invariably organic peroxides. Since these are

chemically unstable as a class of compounds, of which some can decompose

explosively in the pure form, they are mostly supplied as solutions,

dispersions pastes or powder. Most organic peroxides are used at between 1

and 4% based on the resin weight.

Organic peroxide can be subdivided into two broad classes-true

peroxides (benzoyl peroxide) and hydro peroxides (cumene peroxide). Methyl

ethyl ketone peroxide (MEKP) is considered to be mixed peroxide. The most

commonly encountered peroxide catalyst for room temperature cure are

MEKP, cyclohexanone peroxide and acetyl acetone peroxide.

For elevated temperature cure benzoyl peroxide is most frequently

used. There are, however, numerous other peroxides available for specific

applications and specific temperature ranges. When selecting any catalyst,

consideration must be given to cure temperature and gel time with the

particular resin to be used. Shelf life of the catalyst system is also of

importance. Shelf life can be improved by adding inhibitors.

1.4.5.2 Accelerator

The most commonly used accelerator is either those based on

cobalt soap or those based on a tertiary amine. For specific applications mixed

cobalt/tertiary amine accelerators are used to give very short gel time.

33

Accelerators are usually used at between 0.5 and 4% based on the resin

weight. Cobalt octoate is an extremely active and most widely used

accelerator for curing the unsaturated polyester resin.

1.5 CALCIUM CARBONATE

Calcium carbonate is a chemical compound with the chemical

formula CaCO3. Calcium carbonate is readily available in all continents and

its use in the plastics industry in much greater that other filler. The reasons for

popularity of calcium carbonate in plastics are its ready availability, good

color and low cost, together with its favourable particle shape which doesnt

increase the polymer viscosity excessively or lower the strength and impact

resistance significantly.

About 65% of the calcium carbonate filler in plastics is used in

PVC, where it promote fusion and meets the requirement of important end

products at moderate cost. A further 20% is used in unsaturated polymer

thermoset, and much of the rest is targeted at PP and PE, almost all

thermoplastics and thermosetting polymer use it to some extent as filler.

It is widely used as extended filler and has a crystal structure of

trigonal calcite (Figure 1.5). It is a common substance found in rock (Chalk,

Limestone, Marble and Travertine) in all parts of the world and is the main

component of shells of marine organisms, snails, pearls and eggshells.

Calcium carbonate is the active ingredient in agricultural lime and is usually

the principal cause of hard water.

34

Figure 1.5 Crystal structure of calcite

During material compounding it is used to improve impact strength,

aid processing and reduce cost. Another major use of calcium carbonate is

polyester compounds, where calcium carbonate is used for its low oil

absorbtion and dimensional stability during cure, cost saving and contribution

on smooth surface. The surface modified calcium carbonate is used in rigid

pipes for high impact resistance, smooth surface finish, easy processing at less

impact modifier addition.

1.5.1 Preparation

The vast majority of calcium carbonate used in industry is extracted

by mining or quarrying. Pure calcium carbonate (e.g. for food or

pharmaceutical use), can be produced from a pure quarried source (usually

marble).

Alternatively, calcium oxide is prepared by calcining crude calcium

carbonate. Water is added to give calcium hydroxide and carbon dioxide is

passed through this solution to precipitate the desired calcium carbonate,

referred to in the industry as precipitated calcium carbonate.

35

CaCO3 CaO + CO2

CaO + H2O Ca(OH)2

Ca (OH) 2 + CO2 CaCO3 + H2O

1.5.2 Chemical Properties

Calcium carbonate shares the typical properties of other carbonates.

Notably:

it reacts with strong acids, releasing carbon dioxide:

CaCO3 + 2HCl CaCl2 + CO2 + H2O

it releases carbon dioxide on heating (to above 840 C in the

case of calcium carbonate), to form calcium oxide, commonly

called quicklime, with reaction enthalpy 178 kJ/mole:

CaCO3 CaO + CO2

Calcium carbonate will react with water that is saturated with

carbon dioxide to form the soluble calcium bicarbonate:

CaCO3 + CO2 + H2O Ca(HCO3)2

1.5.3 Applications

The main use of calcium carbonate is in the construction industry,

either as a building material in its own right (e.g. marble) or limestone

aggregate for road building or as an ingredient of cement or as the starting

material for the preparation of builder's lime by burning in a kiln.

Calcium carbonate is also used in the purification of iron from iron

ore in a blast furnace. Calcium carbonate is calcined in situ to give calcium

oxide, which forms a slag with various impurities present and separates from

the purified iron.

36

In the oil industry, calcium carbonate is used in drilling fluids and

filter cake sealing agent. It may also be used as a weighting material to

increase the density of drilling fluids to control down hole pressures. Calcium

carbonate is widely used as an extender in paints, in particular matte emulsion

paint where typically 30% by weight of the paint is either chalk or marble.

Calcium carbonate is also widely used as filler in plastics. Some

typical examples include around 15 to 20% loading of chalk in unplasticized

polyvinyl chloride drain pipe, 5 to 15% loading of stearate coated chalk or

marble in unplasticized polyvinyl chloride window profile. Polyvinyl chloride

cables can use calcium carbonate at loadings of up to 70 phr (parts per

hundred parts of resin) to improve mechanical properties (tensile strength and

elongation) and electrical properties (volume resistivity). It is also routinely

used as filler in thermosetting resins (sheet and bulk molding compounds).

Calcium carbonate is also used in a wide range of trade like do it yourself

adhesives, sealants and decorating fillers. Ceramic tile adhesives typically

contain 70 to 80% limestone.

Ground calcium carbonate or precipitated calcium carbonate is used

as filler in paper. Ground calcium carbonate and precipitated calcium

carbonate are cheaper than wood fiber, so adding it to paper is cost efficient

for the paper industry. Printing and writing paper can be made of 10-20%

calcium carbonate. Calcium carbonate is used in the production of toothpaste.

Also, it has seen resurgence as a food preservative and color retainer, when

used in or with products such as organic apples or food.

1.6 SILICON DIOXIDE

The chemical compound silicon dioxide, also known as silica (from

the Latin silex), is an oxide of silicon with the chemical formula of SiO2.

37

Silica is most commonly found in nature as sand or quartz, as well as in the

cell walls of diatoms. Silica is the most abundant mineral in the Earth's crust.

Silica is used as filler in plastics. Silica gives good balance of

properties as filler in casting resin such as unsaturated polyester, epoxies,

vinyl ester, offering good dimensional stability, good electrical properties,

abrasion resistance, scratch resistance and thermal conductivity, together with

the opportunity in cost reduction. Its main properties are irregular particle

shape, low cost and high purity.

Silica products are diverse groups and fall into three groups.

Precipitated silica, diatomaceous silica and ground silica. Precipitated silica is

used primarily as thixotropes and viscosity control agents in unsaturated

polyester. Precipitated silica is also used as a processing aid in extrusion

molding of various resins. Silica fillers are used for obtaining hardness,

strength, chemical resistance, flow, electrical insulation, thermal conductivity,

heavy loading, dimensional stability and wear resistance.

1.6.1 Crystal Structure

In the vast majority of silicates, the Si atom shows tetrahedral

coordination, with four oxygen atoms surrounding a central Si atom. The

most common example is seen in the quartz crystalline form of silica. In each

of the most thermodynamically stable crystalline forms of silica, on average,

only two out of four of each vertices (or oxygen atoms) of the SiO4 tetrahedra

are shared with others, yielding the net chemical formula: SiO2.

1.6.2 Applications

Silica is used as semi reinforcing fillers in thermoplastics,

thermosets and elastomer, as antiblocking additives for films, as viscosity

38

regulators and as matting agent. Other application includes plastisols,

adhesive, coating, sealants and electric components. Silica is frequently

treated with coupling agent, preferably silanes. Special surface coating for

silica, e.g. polyisoprene (for styrene butadiene rubber), polyethylene (for

nucleation), polyperoxides (as cross linking agents) and organic pigments

have been developed and marketed.

Silica is used primarily in the production of window glass, drinking

glasses and bottled beverages. The majority of optical fibers for

telecommunications are also made from silica. It is a primary raw material for

many whiteware ceramics such as earthenware, stoneware and porcelain, as

well as industrial portland cement.

In electrical applications, it can protect the silicon, store charge,

block current and even act as a controlled pathway to limit current flow.

In pharmaceutical products, silica aids powder flow when tablets

are formed. Finally, it is used as a thermal enhancement compound in ground

source heat pump industry.

1.7 ALUMINA

Alumina has a chemical formula of Al2O3 and is the most cost

effective and widely used material in the family of engineering ceramics.

Alumina is well known as the large volume flame retardant used in the world,

with a consumption of around 2, 00,000 tons per annum. Alumina is a

moderately priced filler. It is a halogen free flame retardant and a smoke

suppressant, especially in electrical cable industry and in thermosetting

compounds for the building industry. When exposed to high temperature,

alumina gives off water, thereby reducing the flame spread. In some resin

39

systems, such as unsaturated polyester, it is used as extender filler replacing

more expensive resin in the formulation.

The use of alumina as a nanomaterial for reinforcement is limited.

Incorporation of nano alumina improves properties like hardness, wear

resistance, dielectric properties, resists strong acid and alkali attack at

elevated temperatures, good thermal conductivity, size and shape capability,

high strength and stiffness, etc. With this reasonably good combination of

properties and an attractive price, nano alumina filled composites has a very

wide range of applications.

1.7.1 Natural Occurrence

Corundum is the most common naturally occurring crystalline form

of aluminium oxide. Much less-common rubies and sapphires are gem-quality

forms of corundum, which owe their characteristic colors to trace impurities

in the corundum structure.

1.7.2 Production

Aluminium hydroxide minerals are the main components of

bauxite, the principal ore of aluminium.

Bauxite is purified by the Bayer process:

Al2O3 + 3H2O + 2NaOH 2NaAl(OH)4

The silica dissolves as silicate Si(OH)62-

. Upon filtering, Fe2O3 is

removed. When the Bayer liquor is cooled, Al (OH)3 precipitates, leaving the

silicates in solution. The mixture is then calcined (heated strongly) to give

aluminium oxide:

2Al(OH)3 Al2O3 + 3H2O

40

The formed Al2O3 is alumina. The alumina formed tends to be

multi-phase; i.e., constituting several of the alumina phases rather than solely

corundum. The production process can therefore be optimized to produce a

tailored product. The type of phases present affects, for example, the

solubility and pore structure of the alumina product which, in turn, affects the

cost of aluminium production and pollution control.

1.7.3 Properties

Aluminium oxide is an electrical insulator but has a relatively high

thermal conductivity (40 Wm1K

1) for a ceramic material. Its hardness

makes it suitable for use as an abrasive and as a component in tools. It has

density of 2.42 g/cm3. The Mohs hardness is 2.5-3.5. The refractive index is

1.58, which is similar to the polymer resin. It is quite stable at 220 C.

1.7.4 Applications

Fiber reinforced polyester: Construction related end uses such

as bath tubs, shower stalls, panels and skylights. It also acts as a

resin extender

SMC/BMC laminates: Mostly for electronic equipments as well

as appliance and automotive parts

Flexible polyurethane foams for seating and mattresses

Used for encapsulation, potting and epoxy glass laminated for

electrical/electronic uses

Styrene butadiene rubber (SBR) latex foams and adhesive for

carpet backing

41

Cross linked polyethylene, cross linked ethylene vinyl acetate

and ethylene propylene diene monomer for wire and cable

insulation and jacketing

Styrene butadiene rubber mechanical goods, such as mine

belting

Coating, paints, adhesive and sealants for flame retardance

Flexible polyvinyl chloride for wall coverings, upholstery and

wire and cable insulation

Annual world production of alumina is approximately 45 million

tonnes. Over 90% of which is used in the manufacture of aluminium metal.

The major uses of specialty aluminium oxides are in refractories, ceramics

and polishing and abrasive applications. Large tonnages are also used in the

manufacture of zeolites, coating titania pigments and as a fire retardant/

smoke suppressant. Aluminium oxide is also used in the preparation of

coating suspensions in compact fluorescent lamps.

Health and medical applications include it as a material in hip

replacements. It is also used in toothpaste formulations. Most pre-finished

wood flooring now uses aluminium oxide as a hard protective coating. It is

widely used in the fabrication of superconducting devices.

1.8 ZINC OXIDE

Zinc oxide is an inorganic compound with the formula ZnO. It

usually appears as a white powder, nearly insoluble in water. Zinc oxide is

present in the Earths crust as the mineral zincite; however, most zinc oxide

used commercially are produced synthetically. Nano zinc oxide, as one of the

multifunctional inorganic nanoparticles, has drawn increasing attention in

recent years due to its prominent physical and chemical properties, such as

42

chemical stability, low dielectric constant, high luminous transmittance, high

catalysis activity, effective antibacterial and bactericide, intensive ultraviolet

and infrared absorption. Moreover, the advance of nano zinc oxide particles

could improve the mechanical, thermal and optical properties of the polymer

matrix.

Zinc compounds can provide a variety of properties in the plastics

field. Zinc oxide reacts with unsaturated polyesters to form higher viscosity

and a thixotropic body. The formulation of polyesters in the presence of zinc

oxide increases the viscosity only to a limited extent unlike magnesium oxide

which increases the viscosity tremendously even at lower contents.

1.8.1 Crystal Structures

Zinc oxide crystallizes in three forms: hexagonal wurtzite, cubic

zinc blende and the rarely observed cubic rocksalt. The wurtzite structure is

most stable and thus most common at ambient conditions.

(a) Wurtzite structure (b) Zinc blende structure

Figure 1.6 Crystal structures of (a) Wurtzite and (b) Zinc blende

structures of zinc oxide

The zinc blende form can be stabilized by growing zinc oxide on

substrates with cubic lattice structure. In both cases, the zinc oxide is

43

tetrahedral. The rocksalt NaCl-type structure is only observed at relatively

high pressures - ~10 GPa.

1.8.2 Properties

The physical properties of zinc oxide are depicted in Table 1.4.

Table 1.4 Properties of zinc oxide

Molecular formula ZnO

Appearance White solid

Density 5.606 g/cm3

Melting point 1975 C (decomposes)

Boiling point 2360 C

Solubility in water 0.16 mg/100 mL (30 C)

Zinc oxide occurs as white powder known as zinc white or as the

mineral zincite. The mineral usually contains a certain amount of manganese

and other elements and is of yellow to red color. Crystalline zinc oxide is

thermochromic, changing from white to yellow when heated and in air

reverting to white on cooling. It is nearly insoluble in water and alcohol but it

is soluble in (degraded by) most acids, such as hydrochloric acid.

ZnO + 2HCl ZnCl2 + H2O

Zinc oxide decomposes into zinc vapor and oxygen only at around

1975 C, reflecting its considerable stability. Heating with carbon converts the

oxide into the metal, which is more volatile than the oxide.

ZnO + C Zn + CO

44

It reacts with hydrogen sulfide to give the zinc sulfide: this reaction

is used commercially in removing H2S using zinc oxide powder (e.g., as

deodorant).

ZnO + H2S ZnS + H2O

1.8.3 Applications

The applications of zinc oxide powder are numerous and the

principal ones are summarized below. Most applications exploit the reactivity

of the oxide as a precursor to other zinc compounds. Consequently, it is added

into various materials and products, including plastics, ceramics, glass,

cement, rubber, lubricants, paints, ointments, adhesive, sealants, pigments,

food, batteries, fire retardants, etc.

Rubber manufacture

About 50% of the zinc oxide is used in rubber industry. Zinc oxide

is also an important additive to the rubber used in car tyres. Zinc oxide along

with stearic acid activates vulcanization. Vulcanization catalysts are derived

from zinc oxide and it considerably improves the thermal conductivity.

Concrete industry

Zinc oxide is widely used for concrete manufacturing. Addition of

zinc oxide improves the processing time and the resistance of concrete against

water.

Medical field

Zinc oxide is widely used to treat a variety of skin conditions, in

products such as baby powder and barrier creams to treat diaper rashes,

calamine cream, anti-dandruff shampoos and antiseptic oinments.

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Cigarette filters

Zinc oxide is a constituent of cigarette filters for removal of

selected components from tobacco smoke. A filter consisting of charcoal

impregnated with zinc oxide and iron oxide removes significant amounts of

HCN and H2S from tobacco smoke without affecting its flavor.

Food additive

Zinc oxide is added to many food products, e.g., breakfast cereals,

as a source of zinc, a necessary nutrient (Other cereals may contain zinc

sulfate for the same purpose). Some prepackaged food also includes trace

amounts of zinc oxide even if it is not intended as a nutrient.

Coatings

Paints containing zinc oxide powder have long been utilized as

anticorrosive coatings for various metals. Plastics such as poly (ethylene-

naphthalate) (PEN) can be protected by applying zinc oxide coating. The

coating reduces the diffusion of oxygen with PEN. Zinc oxide layers can also

be used on polycarbonate (PC) in outdoor applications. The coating protects

polycarbonate from solar radiation and decreases the oxidation rate and

photo-yellowing of polycarbonate.

1.9 LITERATURE REVIEW

1.9.1 Progress in Nano Calcium Carbonate

Chan et al (2002) studied the polypropylene/calcium carbonate

nanocomposites. The differential scanning calorimetric results indicate that

the calcium carbonate nanoparticle is very effective nucleating agent for

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polypropylene. The tensile strength, modulus and stiffness were increased

significantly.

Murgic et al (2002) studied the compatibilizing effect of nano sized

calcium carbonate filler on immiscible blends of styrene co acrylonitrile/

ethylene propylene diene.

Zhang et al (2004) observed better improvement in impact strength

and Youngs modulus in polypropylene matrix by adding nano calcium

carbonate than micro calcium carbonate.

Chen et al (2004) prepared polyvinylchloride/nano calcium

carbonate binary composites and polyvinylchloride/blendex/nano calcium

carbonate ternary composites. They reported that the toughness, vicat

softening temp, flexural modulus, storage modulus and glass transition

temperture have increased by adding calcium carbonate in polyvinylchloride

blends matrix.

Xie et al (2004) observed that addition of nano calcium carbonate

in polyvinylchloride matrix shows an increase in the glass transition

temperature (Tg) and 5% weight loss temperature compared to pristine

polyvinylchloride. At 5 wt% nano particle, optimal properties were obtained

in tensile yield strength, impact strength and Youngs modulus.

Mishra et al (2005) used an elegant approach of in situ deposition

technique for the synthesis of nano calcium carbonate. They concluded that

increment in tensile strength with increases of nano calcium carbonate was

observed and lower particle size showed greater improvement.

Jiang et al (2005) reported that the nanosized calcium

carbonate/acrylonitrile-butadiene-styrene composites showed enhanced

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properties compared to micro sized calcium carbonate/acrylonitrile-butadiene-

styrene composites.

Tian et al (2005) studied the effect of high energy vibromilling on

interfacial interaction and mechanical properties of polyvinyl chloride/nano

calcium carbonate composites. The results revealed that the mechanical

property of polyvinyl chloride/treated nano calcium carbonate is remarkably

improved. Dynamic mechanical thermal analysis result also shows improved

value than pristine polyvinyl chloride.

Mishra and Shimpi (2005) synthesized the nano calcium carbonate

using in situ deposition technique and prepared nano calcium

carbonate/styrene-butadiene rubber composites and found a drastic

improvement in mechanical properties, swelling index, specific gravity, flame

retadency and abrasion resistance indices than commercial calcium carbonate

and fly ash filled styrene-butadiene rubber.

Lazzeri et al (2005) showed that Youngs modulus of polyethylene

can be promoted by about 70% as 10 vol% nanosized calcium carbonate was

added to it.

Yang et al (2006) studied the morphology and mechanical

properties of polypropylene/calcium carbonate nanocomposites. The results

reveal that the impact strength and yield strength of nanocomposite are better

and the toughness of the matrix was increased.

The effects of addition of calcium carbonate to thermoplastic

polymers have been investigated by many researchers. Enhancement of

viscosity (Lazzeri et al 2005), stiffness, dimensional stability, yield stress,

crystallinity and some properties have been reported by addition of calcium

carbonate to polymer matrix (Banhegyi and Karasz 1986). On the other hand,

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the calcium carbonate has unfavourable effects on high density polyethylene

(HDPE) matrix (Wang et al 1997). For example, Sahebian et al (2009)

showed that the addition of nano sized calcium carbonate in high density

polyethylene matrix caused a decrease in the fracture toughness of high

density polyethylene.

Yu et al (2006) studied the different methods for the preparation of

epoxy/calcium carbonate nanocomposites and observed the performance of

resultant powder coating. The toughness of powder coating film was

remarkably improved when nano calcium carbonate was introduced in epoxy

resin by in situ and inclusion polymerization.

Zhou et al (2006) studied the nano calcium carbonate filled

ethylene propylene diene monomer. The results show that methacrylic acid

improves the interfacial interaction between nano calcium carbonate particle

and ethylene propylene diene monomer and showed superior mechanical

properties.

Wu et al (2006) prepared the polymethylmethacrylate/calcium

carbonate nanocomposites by soapless emulsion polymerization technique.

Sahebian et al (2007) illustrated that creep behaviour of

polyethylene nanocomposites reinforced with different nanosized calcium

carbonate depends strongly on calcium carbonate content.

Huang (2007) concluded that polypropylene on blending with nano

calcium carbonate, exhibited greatly enhanced foamability, increase in storage

modulus compared with pure polypropylene.

Wang et al (2007) fabricated the ternary nanocomposites by

polyvinylchloride/vulcanized powder rubber/nanosized calcium carbonate.

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Wang et al (2007) synthesized the nano calcium carbonate and it

was dispersed in low density polyethylene. They observed that the tensile

property and flexural modulus of the system have been evidently increased.

Ma et al (2007) studied the properties of polypropylene by

incorporating ethylene octane copolymer and nano calcium carbonate. They

observed that impact strength of ternary composites was significantly

increased meanwhile stiffness and tensile strength remains unchanged.

Sahebian et al (2008) investigated the effect of nano calcium

carbonate coated with stearic acid on fracture toughness of high density

polyethylene/calcium carbonate nanocomposites. They concluded that

fracture toughness increased slightly as stearic acid content increased.

Wu et al (2008) synthesized the ultra fine particles of calcium

carbonate by dispersing the mixture of carbon dioxide and nitrogen into the

calcium hydroxide/water slurry with a microspore plate.

Jin et al (2008) studied the mechanical and interfacial properties of

trifunctional epoxy resin triglycidyl para aminophenol/calcium carbonate

nanocomposites. They concluded that the mechanical properties and fracture

toughness were significantly increased with the addition of nano calcium

carbonate.

Jin et al (2008) studied the thermo mechanical behaviour of

butadiene rubber reinforced with nano calcium carbonate. They concluded

that thermal stability and mechanical properties increased by addition of nano

calcium carbonate, while glass transition temperature remains constant.

Ma et al (2008) synthesized the polymethylmethacrylate/calcium

carbonate nanocomposites by soapless emulsion polymerization method. The

50

results indicate that the calcium carbonate nanoparticles improve the thermal

stability and acid resistance of polymethylmethacrylate.

Wang et al (2008) studied the effect of nano calcium carbonate

particles on the mechanical properties of nano calcium carbonate/

acrylonitrile-butadiene-styrene composites. The impact strength of composite

materials was enhanced.

Sahebian et al (2009) fabricated the high density polyethylene /10%

calcium carbonate nanocomposites by twin screw extruder. The differential

scanning calorimetric results show enhancement of heat capacity, sensible

heat and crystallinity index.

Jain et al (2009) prepared the epoxy/calcium carbonate

nanocomposites by using diaminodiphenyl sulfone as curing agent. The

differential scanning calorimetric studies show that heat of curing decreases

with increasing amount of nano calcium carbonate. The thermogravimetry

showed that all samples are stable up to 350 C.

Zhang et al (2010) studied the mechanical properties of

acrylonitrile-butadiene-styrene/polymethylmethacrylate/calcium carbonate

nanocomposites. The tensile and impact strength of treated calcium carbonate

nanocomposites are superior compared to that of untreated calcium carbonate

nanocomposites.

1.9.2 Progress in Nano Silica

Rong et al (2001) examined the effect of grafted nano silica into

polypropylene composites. Mechanical tests indicated that all the composites

incorporated with grafted silica particle possess much greater impact strength

than untreated silica/polypropylene composites.

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Li et al (2002) studied the effect of silane treatment on preparation

of nylon 6/silica nanocomposites through in situ polymerization. However,

the dynamic mechanical analysis and mechanical test revealed that treating

silica with silane improved the strength and toughness of composite materials.

Xiong et al (2002) studied the preparation and characterization of

acrylic latex/silica nanocomposites. The composites were prepared via high

shear stirring, ball milling and in situ polymerization. The results show that

shear stirring and ball milling methods could give better nanocomposites than

the in situ method.

Zhang et al (2003) studied the effect of nano silica filler on the

rheological and morphological properties of polypropylene/liquid crystalline

polymer blends.

Zhang et al (2003) observed that incorporation of nano silica

particle that are pretreated by graft polymerization into high density

polyethylene is an effective way to improve mechanical properties of matrix.

Such improvement can be acquired at nano silica content as low as 0.75 vol%.

Chen et al (2003) synthesized nanocomposites by the polyurethane

based on polyester/silica. Results show that silica nanoparticle in

polyurethane enhanced the hardness, glass transition temperature and

adhesion strength.

Rong et al (2004) analyzed the interfacial interaction in

polypropylene/silica nanocomposites. They concluded that the greatest

interaction between the modified nanoparticle and the matrix was achieved in

the case of low silica concentration and low percentage of grafting.

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Le et al (2004) synthesized silica hollow spheres with meso

structured walls using nanosized calcium carbonate particles.

Li et al (2005) studied the particle size and morphology of poly

(styrene-co (butyl acrylate)/nano silica composite latex.

Che et al (2005) studied the grafted silica into polybutylene

terephthalate matrix. The results show that the thermal stability, mechanical

properties including tensile strength, elongation at break, notched impact

strength were greatly improved up to silica loading of 2 wt%.

Schulte et al (2005) developed a method for the synthesis of both

spherically micro/nano silica particles and silica hybrid particles using inverse

sol-gel suspension technique.

Liu et al (2005) blended linear low density polyethylene into low

density polyethylene at fixed ratio and filled with nanoparticles of silica and

titanium dioxide at a ratio of up to 5 wt%. The results show that the

mechanical and thermal properties and flexibility of linear low density

polyethylene can further be improved by incorporation of nano particles of

silica and titanium dioxide as fillers.

Kinloch et al (2005) reported the substantial increases in toughness

that may be achieved when nano silica particles are well dispersed in a hot

cured epoxy polymer.

Sreekala et al (2005) incorporated the silica nano particles into

reactive epoxy resin via sol-gel process. The results show that nano particle

had no significant effect on the viscosity and optical property of the system.

They found that at volume content up to 14%, the particle causes an

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enhancement of mechanical properties, including bending strength and

modulus, micro hardness, fracture toughness and sliding wear resistance.

Jain et al (2005) developed a new route by grafting vinyl

triethoxysiliane by sol-gel method to prepare polypropylene/silica

nanocomposites.

Jongsomjit et al (2005) studi